1. Field of the Invention
The present invention relates to a coating structure for improving the corrosion resistance of aluminum alloy-made products and parts used under a water environment, such as marine screws and hulls used on sea water or a lake, water pumps and sprayers having a power such as a general-purpose engine, etc., and agricultural working machines used in a paddy field, etc.
2. Description of the Related Art
To the products and parts described above is applied rust-preventing or corrosion-resisting coating. In particular, rust-preventing coating to sea water containing salinity which is a factor of accelerating corrosion is required.
Concerning rust-preventing or corrosion-resisting coating, many techniques are proposed. For example, Japanese Patent Laid-Open Publication No. HEI-2-250997 discloses a "RUST-PREVENTING TREATMENT METHOD FOR ALUMINUM MATERIAL AND ALUMINUM-MADE OUTBOARD MOTOR FRAME" obtained by forming an anodized film on the surface of a material composed of aluminum or an aluminum alloy and subjecting the anodized film to a sealing treatment with molybdenum disulfide to form thereon a coated film. In the above-mentioned publication, it is described that in the coating structure it is desirable to first coat a primer containing a rust-preventing pigment prior to forming the coated film and also as the rust-preventing pigment which is mixed with the primer, strontium chromate is suitable.
Also, Japanese Patent Laid-Open Publication No. HEI-10-230219 discloses a "COATED FILM STRUCTURE WITH EXCELLENT CORROSION RESISTANCE TO SEAWATER" obtained by forming a formation film on the surface of an aluminum part 11 by a chromate treatment, forming an under coated layer on the surface of the formation film with a rust-preventing pigment coating material using zinc phosphate, and coating thereon a topcoat material.
However, in the coating structure of the Japanese Publication No. HEI-2-250997 described above, a coating anodizing treatment (anodized aluminum treatment) and a sealing treatment are required and these treatments increase the cost, which causes the cost up of the products.
Furthermore, because in the coated film of the Japanese Publication No. HEI-10-230219 described above, zinc phosphate is used, it is anticipated that the strength of the coated film is lowered. If the strength of the coated film is lowered, the corrosion resistance is lowered.
Particularly, because in both Japanese Patent Laid-Open Publication No. HEI-2-250997 and No. HEI-10-230219 described above, a chromic acid treatment or a chromic acid-base primer is used, the treatment of a chromic acid-base waste solution becomes inevitable and the cost for the waste solution treatment becomes a factor of increasing the product cost.